Spectroelectrochemically determined energy levels of PM6:Y6 blends and their relevance to solar cell performance

Abstract

A systematic spectroelectrochemical approach is presented to precisely determine frontier orbital energies of PM6:Y6 blends in device-relevant films and results are discussed regarding their impact on solar cell performance.

Impact of the Molecular Structure of Oligo(ethylene glycol)-Incorporated Y-Series Acceptors on the Formation of Alloy-like Acceptors and Performance of Non-Halogenated Solvent-Processable Organic Solar Cells

“…The mixtures contained Y6 and Y- n C-4O at a weight ratio of 80:20, which is the ratio that optimized the performance of the resulting OSC. The solubility of Y6 in o -xylene was only 10.5 mg mL –1 at room temperature, which was not sufficient to produce an optimally thick active layer. , Interestingly, the addition of Y- n C-4O guest SMAs significantly improved the processability of Y6 in o -xylene. In detail, with 20 wt % addition of the guest SMAs, the solubility of the Y6:Y- n C-4O mixture significantly increased to 41.5 mg mL –1 for Y6:Y-4C-4O, 42.3 mg mL –1 for Y6:Y-6C-4O, and 46.8 mg mL –1 for Y6:Y-12C-4O.…”

What is special about Y6; the working mechanism of neat Y6 organic solar cells

“…15 Such small values are debatable compared to cyclic voltammetry measurements where exciton binding energy of around 0.3 eV can be estimated. 16 Nevertheless, all of these values are larger than thermal energy, k B T, which make spontaneous efficient charge generation implausible. In addition, another drawback of neat devices is the difficulty obtaining the optimized phase separation between donor (D) and acceptor (A) segments in a single material, resulting in severe charge recombination and low charge transport efficiency, as reported by Hodgkiss and co-workers.…”

“…15 Such small values are debatable compared to cyclic voltammetry measurements where the exciton binding energy of around 0.3 eV can be estimated. 16 In addition, another drawback of neat devices is the difficulty in obtaining the optimized phase separation between donor (D) and acceptor (A) segments in a single material, resulting in severe charge recombination and low charge transport efficiency, as reported by Hodgkiss and co-workers. 12 To reflect the importance of all of these exhilarating findings, we fabricated single component Y6 devices with an efficiency of 4.5%-the highest PCE reported for single material small molecule OSCs.…”

Elucidating How Low Energy Offset Matters to Performance of Nonfullerene Acceptor-Based Solar Cells

“…In this study, we blended the donor polymer PM6 (full compound names given in Supplementary Note 1) with three different small-molecule acceptors: Y6, ITIC, and o-IDTBR (Figure 1a−d). Figure 1e demonstrates the energy levels of the neat materials as well as blend films obtained using cyclic voltammetry (see Figure S1 for cyclic voltammetry data, Supplementary Note 2 for measurement details, and Supplementary Note 3 for comment; PM6 and Y6 energy levels were also reported by our group earlier 9 ). The resulting HOMO−HOMO offsets range from −30 to ca.…”

“…Organic solar cells (OSCs) have seen a significant improvement in performance since the introduction of nonfullerene acceptors (NFAs) as one of the components of the bulk heterojunction (BHJ) layer. , The ability to control the synthetic routes and, therefore, structure of these molecules yields a variety of important properties, including solubility, film morphology, charge carrier mobility, and energetics. , The energetic aspect is particularly important for a BHJ, as it affects charge generation across the interface between the donor (D) and the acceptor (A) materials. , Whereas electron transfer from D to A has been previously regarded as the main mechanism of charge generation, the current general consensus underscores the predominant role of hole transfer from A to D in NFA-based blends due to the typically smaller acceptor bandgap and a considerable exciton contribution from the acceptor. In this picture, the offset between the highest occupied molecular orbital (HOMO) levels of the two blend components should play a critical role in charge generation, as it represents a driving force for the transition from a singlet exciton (S 1 ) to a charge transfer (CT) state. , While earlier work reported efficient charge generation for vanishing HOMO offset, it is now well-established that below a critical value ranging, according to various reports, from below 0.2 eV to ca. 0.5 eV the exciton dissociation efficiency decreases continuously with decreasing offset. ,, This has been related to a decrease in the short-circuit current ( J SC ), which overcompensates the increase in open-circuit voltage ( V OC ) with regard to device efficiency.…”